Delayed handover signalling in a mobile network

ABSTRACT

In a mobile network comprising a plurality of base stations and a gateway, a user equipment may perform a handover from a source base station to a target base station. By control signalling, the handover may be notified from the target base station to a control node of the mobile network and then to the gateway so that the gateway may switch to a data path which connects the target base station and the gateway. The notification of the handover may be delayed at the target base station or at the control node, by starting a timer and sending the control signalling for initiating the path switching after expiry of the timer.

TECHNICAL FIELD

The present invention relates to methods for handover signalling in amobile network and to corresponding devices.

BACKGROUND

In mobile networks, e.g., as specified by the 3^(rd) GenerationPartnership Project (3GPP), it is known to perform a handover in theRadio Access Network (RAN) and then inform a core network of theexecuted handover. For example 3GPP Technical Specification (TS) 23.401defines that in a mobile network according to the specifications of 3GPPLTE (Long Term Evolution), a handover is executed in the LTE RAN, usingsignalling between the user equipment (UE), and the involved basestations, referred to as source eNB (evolved Node B) and target eNB.Once the handover is executed and the UE connects to the target eNB, thetarget eNB notifies the handover to the core network, referred to as EPC(Evolved Packet Core). This is accomplished by signalling to a controlnode referred to as MME (Mobility Management Entity). The MME thenhandles switching to a user plane data path which connects the targeteNB with a serving gateway (SGW).

The above handover mechanism may result in significant signalling loadat the MME and/or the SGW. This is particularly true for deploymentswhere the number of handovers is high due to the small cells and highmobility of UEs, or due to the sensitive handover threshold settings inthe RAN which result in handovers even at a slight change in themeasured radio link characteristics.

Accordingly, there is a need for solutions which allow for efficienthandover signalling in a mobile network.

SUMMARY

According to an embodiment of the invention, a method of signalling ahandover of a UE in a mobile network is provided. According to themethod, a node of the mobile network starts a timer. This isaccomplished in response to a handover of the UE from a source basestation to a target base station. After expiry of the timer, the nodesends control signalling for initiating switching to a data path whichconnects the target base station and a gateway of the mobile network.

According to a further embodiment of the invention, a node of a mobilenetwork is provided. The node comprises an interface for signalling ahandover of a UE and a processor. The processor is configured to start atimer in response to a handover of the UE from a source base station toa target base station. Further, the processor is configured to controlthe interface to send, after expiry of the timer, control signalling forinitiating switching to a data path which connects the target basestation and the gateway.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a handover scenario according to anembodiment of the invention.

FIG. 2 shows a timing diagram for illustrating exemplary proceduresinvolving handover signalling according to an embodiment of theinvention.

FIG. 3 schematically illustrates a further handover scenario accordingto an embodiment of the invention.

FIG. 4 schematically illustrates a further handover scenario accordingto an embodiment of the invention.

FIG. 5 shows a timing diagram for illustrating further exemplaryprocedures involving handover signalling according to an embodiment ofthe invention.

FIG. 6 shows a timing diagram for illustrating further exemplaryprocedures involving handover signalling according to an embodiment ofthe invention.

FIG. 7 shows a timing diagram for illustrating further exemplaryprocedures involving handover signalling according to an embodiment ofthe invention.

FIG. 8 shows a timing diagram for illustrating further exemplaryprocedures involving handover signalling according to an embodiment ofthe invention.

FIG. 9 shows a timing diagram for illustrating exemplary procedures forforwarding of control signalling according to an embodiment of theinvention.

FIG. 10 shows a timing diagram for illustrating exemplary proceduresaccording to an embodiment of the invention, in which path switching istriggered by a need for uplink control signalling.

FIG. 11 shows a timing diagram for illustrating exemplary proceduresaccording to an embodiment of the invention, in which path switching istriggered by a need for downlink control signalling.

FIG. 12 shows a timing diagram for illustrating exemplary procedures foravoiding forwarding loops according to an embodiment of the invention.

FIG. 13 shows a timing diagram for illustrating further exemplaryprocedures for avoiding forwarding loops according to an embodiment ofthe invention.

FIG. 14 shows a timing diagram for illustrating exemplary procedures forreleasing a connection according to an embodiment of the invention.

FIG. 15 shows a flowchart for illustrating a method according to anembodiment of the invention.

FIG. 16 schematically illustrates a control node according to anembodiment of the invention.

FIG. 17 schematically illustrates a base station according to anembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, the invention will be explained in more detail byreferring to exemplary embodiments and to the accompanying drawings. Theillustrated embodiments relate to handover signalling in a mobilenetwork. In the illustrated examples, it is assumed that the mobilenetwork is implemented on the basis of 3GPP LTE radio access technology.However, it is to be understood that other types of radio accesstechnology could be used as well, e.g., Global System for MobileCommunications (GSM) or Universal Mobile Telecommunications System(UMTS).

FIG. 1 schematically illustrates a handover scenario in the mobilenetwork. More specifically, FIG. 1 illustrates a RAN 100 and a corenetwork 300 of the mobile network. The RAN includes a first base station(BS) 110 and a second BS 120. The core network 300 includes a controlnode 320 and a gateway 350. In accordance with the illustrated LTEscenario, the first BS 110 and the second BS 120 are implemented aseNBs, the control node 320 is implemented as MME, and the gateway 350 isimplemented as SGW.

Further, FIG. 1 illustrates a UE 10. The UE 10 may be a portablecommunication device, such as a mobile phone, portable computer, or thelike. The UE 10 may connect to the first or second BS 110, 120 so as totransmit user plane data to a packet network connected to the gateway350 and/or receive user plane data from the packet network. If the UE 10is connected to the first BS 110, typically a first data path P1 betweenthe first BS 110 and the gateway 350 is used for conveying the userplane data to or from the packet network. If the UE 10 is connected tothe second BS 120, typically a second data path P2 between the second BS120 and the gateway 350 is used for conveying the user plane data to orfrom the packet network. In the illustrated LTE scenario, the first datapath P1 may be implemented using a S1-U interface between the first BS110 and the gateway 350, and the second data path P2 may be implementedusing a S1-U interface between the second BS 120 and the gateway 350.For conveying the user plane data between the UE 10 and the first BS 110or the second BS 120, a radio interface is used, in the illustrated LTEscenario referred to as Uu interface.

Further, the first BS 110 and the second BS 120 may also perform controlsignalling with respect to the control node 320, i.e., transmit controldata to the control node 320 and receive control data from the controlnode 320. For conveying the control plane data, an S1-MME interfacebetween the first BS 110 and the control node 320 and an S1-MMEinterface between the second BS 120 and the control node 320 may beused.

The first BS 110 and the second BS 120 may also communicate with eachother using an inter-BS interface, in the illustrated LTE scenarioreferred to as X2 interface. The inter-BS interface may be used forconveying user plane data and/or control data.

In the scenario of FIG. 1, a situation after a handover of the UE 10from the first BS 110 to the second BS 120 is illustrated. Concerningthis handover, the first BS 110 may be referred to as a source BS, andthe second BS 120 may be referred to as a target BS. Having executed thehandover, the mobile network may switch from using the first data pathP1 for conveying the user plane data to using the second data path P2for conveying the user plane data. After executing the handover, thispath switching is initiated by control signalling between the target BS110 and the control node 320, and by control signalling between thecontrol node 320 to the gateway 350. After executing the handover, butbefore switching to the second path P2, the mobile network may continueusing the first data path for conveying the user plane data. For thispurpose, the user plane data may be forwarded between the first BS 110and the second BS 120, using the inter-BS interface.

According to concepts as illustrated in the following, controlsignalling for initiating the path switching is delayed for a certaindelay time period T, using a corresponding timer. The delay time periodT may be configurable, e.g., using operations and maintenance (OAM)procedures. In this way, the gateway 350 is not notified of everyhandover and the signalling load between the RAN 100 and the corenetwork 300 and/or within the core network 300 may be reduced. Inparticular, if there are multiple handovers during the delay time periodT, control signalling between the target BS to the control node 320and/or between the control node and the gateway 350 only needs to beperformed for the last handover. Further, if the UE 10 returns to thesource BS of the first handover, also control signalling for the lasthandover can be omitted. Accordingly, the concepts allow for hidingcertain handovers from the control node 320 and/or from the gateway 350,thereby improving efficiency of control signalling.

In some implementations, the delay and the corresponding timer may beimplemented in the control node 320. In this case, the target BS maynotify the handover to the control node 320. After expiry of the timer,the control node 320 may then notify the gateway 350. This reduces thesignalling load between the control node 320 and the gateway 350. Inthis case, downlink user plane data may be forwarded from the source BSto the target BS until the path switching is executed. Control planedata between the RAN 100 and the core network 300 may in turn beconveyed using the interface between the target BS and the control node320.

In some implementations, the delay and the corresponding timer may beimplemented in the RAN 100, in particular in the target BS. In thiscase, the target BS notifies the handover to the control node afterexpiry of the timer. The control node 320 may then proceed withnotifying the gateway 350. This reduces the signalling load also betweenthe RAN 100 and the control node 320. In this case, also control planedata may be forwarded between the source BS and the target BS.

FIG. 2 shows a timing diagram for illustrating procedures in a handoverscenario as illustrated in FIG. 1. The procedures involve the UE 10, thesource BS 110, the target BS 120, the control node 320, and the gateway350. In the scenario of FIG. 2, notifying the gateway 350 of a handoverof the UE 10 from the source BS 110 to the target BS 120 is delayed atthe control node 320, and forwarding of downlink user plane data fromthe source BS 110 to the target BS 120 takes place until the gateway 350is notified of the handover and switches to the second data path P2established using the interface between the target BS 120 and thegateway 350.

In the procedures of FIG. 2, the handover from the source BS 110 to thetarget BS 120 is prepared by messages 201 and 202. Message 201 may be ahandover request from the source BS 110 to the target BS 120, andmessage 202 may be a handover request acknowledgement form the target BS120 to the source BS 110, in accordance with 3GPP TS 36.300.

By sending message 203 to the UE 10, the source BS 110 then initiatesexecution of the handover. In accordance with 3GPP TS 36.300, message203 may be a Radio Resource Control (RRC) connection reconfigurationmessage and convey mobility control information to the UE 10. Asillustrated by step 204, the UE 10 may then detach from the cell of thesource BS 110 and synchronize to the cell of the target BS 120. Thesource BS 110 may then send status information to the target BS 120,using message 205.

As illustrated by message 205, the source BS 110 may then startforwarding downlink user plane data to the target BS 120.

By sending message 207 to the target BS 120, the UE 10 then sets up aconnection to the target BS 120. After this, uplink user plane data canbe conveyed directly via the target BS 120 to the gateway 350, asillustrated by messages 208 and 209, while downlink user plane data canbe conveyed from the gateway 350 to the source BS 110 and then forwardedvia the target BS 120 to the UE 10, as illustrated by messages 210 to212.

By sending message 213 to the control node 320, the target BS 120 thenrequests path switching to the second data path P2 established using theinterface between the target BS 120 and the gateway 350. The message 213may be a path switch request in accordance with 3GPP TS 23.401 and36.300. At this point, rather than immediately notifying the gateway350, the control node 320 starts a timer having a duration whichcorresponds to the delay time period T for delaying notification of thehandover to the gateway 350. The duration of the timer, i.e., the delaytime period T, may be configurable, e.g., through OAM procedures. Bymessage 214, the control node 320 acknowledges the path switch requestof message 213.

Upon receiving message 214, the target BS 120 initiates releasing of UEresources at the source BS 110 by sending message 215 to the source BS110. Message 215 may be UE context release message in accordance with3GPP TS 23.401 and 36.300.

Upon receiving message 215, the source BS 110 releases resources relatedto the UE 10, as indicated by step 216. However, the source BS 110 keepssufficient state information to continue forwarding downlink user planedata of the UE 10. In accordance with the illustrated concepts, thesource BS 110 is configured to continue with packet forwarding for aforwarding time period T_(f) which is longer than the delay time periodT for delaying the control signalling to the gateway 350. For thispurpose, a forwarding timer of duration T_(f) may be started togetherwith the forwarding of user plane data by the source BS 110, i.e., uponsending message 206. The forwarding time period T_(f) may beconfigurable, e.g., through OAM procedures. Alternatively oradditionally, the use of the longer forwarding time period T_(f) may benegotiated by the control node 320 and the BS 110 during the setup ofthe connection therebetween. It may also be possible for the controlnode 320 to store information indicating which BS supports asufficiently long forwarding time period T_(f) and to apply the delayingof the handover notification to the gateway only of the source BS 110supports such a sufficiently long forwarding time period T_(f). Thecontrol node 320 may learn such information form the BSs and/or may beconfigured with such information in advance, e.g., on a per TrackingArea Identity (TAI) basis, or on a per network domain network domainbasis.

At step 217, the timer expires. In the scenario of FIG. 2, it is assumedthat the timer expires before the control node 320 receives anindication of the UE 10 having executed a further handover.

After expiry of the timer, the control node 320 notifies the gateway 350of the handover. This is accomplished by sending message 218 to thegateway 350. Message 218 may be a Modify Bearer Request message inaccordance with 3GPP TS 23.401. Upon receiving the message 218, thegateway 350 switches conveying of user plane data to the second datapath P2 of FIG. 1. Accordingly, as illustrated by messages 219 and 220,downlink user plane data are no longer conveyed via the source BS 110.

For facilitating packet reordering, the gateway 350 may then send an endmarker via the source BS 110 to the target BS 120, as illustrated bymessages 221 and 222. The sending of the end marker may be accomplishedin accordance with 3GPP TS 23.401 and 36.300.

By sending message 223 to the control node 320, the gateway 350 thenresponds to the notification of message 218. Message 223 may be a ModifyBearer Response accordance with 3GPP TS 23.401.

At step 224, the source BS 110 stops forwarding of user plane data. Thismay be accomplished in response to detecting the end marker of message221 or upon expiry of the forwarding timer of duration T_(f). Havingstopped forwarding user plane data, the source BS 110 may also releasethe resources associated with the forwarding of user plane data.

FIG. 3 schematically illustrates a further handover scenario. Thescenario of FIG. 3 generally corresponds to that of FIG. 1, but involvesa third BS 130 and a sequence of a first handover from the first BS 110to the second BS 120 and a second handover from the second BS 120 to thethird BS 130. A third data path P3 is established between the third BS130 and the gateway 350. Like in the case of the first data path andsecond data path P2, the third data path may use an S1-U interfacebetween the third BS 130 and the gateway 350. With respect to the firsthandover, the first BS 110 is the source BS and the second BS 120 is thetarget BS. With respect to the second handover, the second BS 120 is thesource BS and the third BS 130 is the target BS. In such a scenario, themultiple handovers of the sequence may be executed within the delay timeperiod T started upon execution of the first handover. In this case,typically only the last handover of the sequence would be notified tothe gateway 350, and forwarding of downlink user plane data may beaccomplished from the source BS of the first handover in the sequence,i.e., the first BS 110, via the target BS of the first handover, i.e.the second BS 120, to the target BS of the last handover in thesequence, i.e., the third BS 130.

In the scenario of FIG. 3, the forwarding of the user plane data betweenthe first BS 110 and the third BS 130 is accomplished by chaining theBSs of the handover sequence. That is to say, each source BS of thesequence forwards downlink user plane data to the its associated targetBS. In the scenario of FIG. 3, the first BS 110 forwards downlink userplane data to the second BS 120 using the inter-BS interface between thefirst BS 110 and the second BS 120, and the second BS 120 forwardsdownlink user plane data to the third BS 130, using an inter-BSinterface between the second BS 120 and the third BS 130. Asillustrated, the inter-BS interfaces may be implemented as X2interfaces.

An alternative arrangement for forwarding the user plane data isillustrated in FIG. 4. In this case, anchoring of the user plane datatraffic in the source BS of the first handover of the sequence is used.This means that the source BS of the first handover of the sequenceforwards the user plane data directly to the target BS of the lasthandover of the sequence, using a corresponding inter-BS interfacetherebetween. In the scenario of FIG. 3, an X2 interface between thefirst BS 110 and the third BS 130 is assumed. However, also other typesof inter-BS interfaces could be used. As compared to the chaining ofFIG. 3, the anchoring of FIG. 4 may provide lower load on the inter-BSinterfaces due to forwarding.

FIG. 5 shows a timing diagram for illustrating procedures in a handoverscenario as illustrated in FIG. 3. The procedures involve the UE 10, thefirst BS 110, the second BS 120, the third BS 130, the control node 320,and the gateway 350. In the scenario of FIG. 5, notifying the gateway350 of the first handover and the second handover is delayed at thecontrol node 320, thereby avoiding notification of the first handover.Forwarding of user plane data between the first BS 110 and the third BS130 is accomplished via the second BS 120 and takes place until thegateway 350 is notified of the second handover and switches to the thirddata path P3 established using the interface between the third BS 130and the gateway 350.

At step 501, the first handover from the first BS 110 to the second BS120 is prepared and executed. This may be accomplished as explained inconnection with messages and steps 201 to 216 of FIG. 2. However, ascompared to FIG. 2 it is now assumed that the second handover from thesecond BS 120 to the third BS 130 is notified to the control node 320before expiry of the timer started upon notification of the firsthandover.

In the procedures of FIG. 5, the handover from the second BS 120 to thethird BS 130 is prepared by messages 502 and 503. Message 502 may be ahandover request from the second BS 120 to the third BS 130, and message503 may be a handover request acknowledgement form the third BS 130 tothe second BS 120, in accordance with 3GPP TS 36.300.

By sending message 504 to the UE 10, the second BS 120 then initiatesexecution of the second handover. In accordance with 3GPP TS 36.300,message 504 may be a RRC connection reconfiguration message and conveymobility control information to the UE 10. As illustrated by step 505,the UE 10 may then detach from the cell of the second BS 120 andsynchronize to the cell of the third BS 130. The second BS 120 may thensend status information to the third BS 130, using message 506.

As illustrated by message 507, the second BS 120 may then startforwarding downlink user plane data to the third BS 130. Here, it is tobe understood that it is the first BS 110 which in turn forwards suchdownlink user plane data to the second BS 120.

By sending message 508 to the third BS 130, the UE 10 then sets up aconnection to the third BS 130. After this, uplink user plane data canbe conveyed directly via the third BS 130 to the gateway 350, asillustrated by messages 509 and 510, while downlink user plane data canbe conveyed from the gateway 350 to the first BS 110 and then forwardedvia the second BS 120 and the third BS 130 to the UE 10, as illustratedby messages 511 to 514.

By sending message 515 to the control node 320, the third BS 130 thenrequests path switching to the third data path P3 established using theinterface between the third BS 130 and the gateway 350. The message 515may be a path switch request in accordance with 3GPP TS 23.401 and36.300. At this point, rather than immediately notifying the gateway350, the control node 320 again restarts the timer with duration T fordelaying notification of the second handover to the gateway 350. Bymessage 516, the control node 320 acknowledges the path switch requestof message 515.

Upon receiving message 516, the third BS 130 initiates releasing of UEresources at the second BS 120 by sending message 517 to the second BS120. Message 517 may be UE context release message in accordance with3GPP TS 23.401 and 36.300.

Upon receiving message 517, the second BS 120 releases resources relatedto the UE 10, as indicated by step 518. However, the second BS 120 keepssufficient state information to continue forwarding downlink user planedata of the UE 10. In accordance with the illustrated concepts, thesecond BS 120 is configured to continue with packet forwarding for aforwarding time period T_(f) which is longer than the delay time periodT for delaying the control signalling to the gateway. For this purpose,a forwarding timer of duration T_(f) may be started together with theforwarding of user plane data by the second BS 120, i.e., upon sendingmessage 507. The forwarding time period T_(f) and the correspondingforwarding timer may be configured in a similar manner as explained inconnection with step 216 of FIG. 2.

Until this point, the gateway 350 was not notified of the first handoveror the second handover because the delay time period of T has notexpired since the last handover notified to the control node 320.Accordingly, forwarding of downlink user plane data as illustrated bymessages 511 to 514 continues.

At step 519, the timer expires. In the scenario of FIG. 5, it is assumedthat the timer expires before the control node 320 receives anindication of the UE 10 having executed a still further handover.

After expiry of the timer, the control node 320 notifies the gateway 350of the second handover. This is accomplished by sending message 520 tothe gateway 350. Message 520 may be a Modify Bearer Request message inaccordance with 3GPP TS 23.401. Upon receiving the message 520, thegateway 350 switches conveying of user plane data to the third data pathP3 of FIG. 3. Accordingly, as illustrated by messages 521 and 522,downlink user plane data are no longer conveyed via the first BS 110 andthe second BS 120.

For facilitating packet reordering, the gateway 350 may then send an endmarker via the first

BS 110 and the second BS 120 to the third BS 130, as illustrated bymessages 523, 524, and 525. The sending of the end marker may beaccomplished in accordance with 3GPP TS 23.401 and 36.300.

By sending message 526 to the control node 320, the gateway 350 thenresponds to the notification of message 520. Message 526 may be a ModifyBearer Response accordance with 3GPP TS 23.401.

At step 527, the first BS 110 stops forwarding of user plane data. Thismay be accomplished in response to detecting the end marker of message523 or upon expiry of the forwarding timer of duration T_(f). Havingstopped forwarding user plane data, the first BS 110 may also releasethe resources associated with the forwarding of user plane data.Similarly, the second BS 120 stops forwarding of user plane data at step528. This may be accomplished in response to detecting the end marker ofmessage 524 or upon expiry of the forwarding timer of duration T_(f).Having stopped forwarding user plane data, the second BS 120 may alsorelease the resources associated with the forwarding of user plane data.

In the example of FIG. 5, the handover related signalling between thecontrol node 320 and the gateway 350 is significantly reduced, becausethe first handover is not notified to the gateway. Instead, only thesecond handover, which followed the first handover within the delay timeperiod T, is notified. It can be seen that even higher reduction ofhandover related signalling could be achieved in the case of longersequences of handovers within the delay time period T. On the otherhand, the notification after the delay time period T is useful in viewof avoiding too complex routing of user plane traffic and reducing loadon the BSs due to traffic forwarding.

A special case of successive handovers during the delay time period Tinvolves the UE 10 returning to the first BS of a sequence of handovers,i.e., to the source BS of the first handover of the sequence. This mayfor example occur for “ping-pong” handovers when the UE 10 moves from afirst BS to a second BS and then back from the second BS to the firstBS. However, this may also occur for longer sequences of handovers inwhich the UE 10 moves to one or more intermediate BSs before returningto the first BS of the sequence.

In such cases, the control node 320 may detect the return of the UE 10to the first BS of the sequence, and then omit notifying the gateway 350of the last handover back to the first BS of the sequence. In this way,handover related signalling between the control node 320 and the gatewaymay be reduced even further.

FIG. 6 shows a timing diagram for illustrating procedures in such aping-pong scenario. The procedures involve the UE 10, the first BS 110,the second BS 120, the control node 320, and the gateway 350, asillustrated in FIG. 1. As compared to FIG. 1, a sequence of handovers isassumed: a first handover of the UE 10 from the first BS 110 to thesecond BS 120, and a second handover of the UE 10 from the second BS 120back to the first BS 110. With respect to the first handover, the firstBS 110 is the source BS and the second BS 120 is the target BS. Withrespect to the second handover, the second BS 120 is the source BS andthe first BS 110 is the target BS. In the scenario of FIG. 6, notifyingthe gateway 350 of the handovers is delayed at the control node 320while forwarding of user plane data is accomplished between the first BS110 and the second BS 120.

In the procedures of FIG. 6, the first handover from the first BS 110 tothe second BS 120 is prepared by messages 601 and 602. Message 601 maybe a handover request from the first BS 110 to the second BS 120, andmessage 602 may be a handover request acknowledgement form the second BS120 to the first BS 110, in accordance with 3GPP TS 36.300.

By sending message 603 to the UE 10, the first BS 110 then initiatesexecution of the handover. In accordance with 3GPP TS 36.300, message603 may be a RRC connection reconfiguration message and convey mobilitycontrol information to the UE 10. As illustrated by step 604, the UE 10may then detach from the cell of the first BS 110 and synchronize to thecell of the second BS 120. The first BS 110 may then send statusinformation to the second BS 120, using message 605.

As illustrated by message 605, the first BS 110 may then startforwarding downlink user plane data to the second BS 120.

By sending message 607 to the second BS 120, the UE 10 then sets up aconnection to the second BS 120. After this, uplink user plane data canbe conveyed directly via the second BS 120 to the gateway 350, asillustrated by messages 608 and 609, while downlink user plane data canbe conveyed from the gateway 350 to the first BS 110 and then forwardedvia the second BS 120 to the UE 10, as illustrated by messages 610 to612.

By sending message 613 to the control node 320, the second BS 120 thenrequests path switching to the second data path P2 established using theinterface between the second BS 120 and the gateway 350. The message 613may be a path switch request in accordance with 3GPP TS 23.401 and36.300. At this point, rather than immediately notifying the gateway350, the control node 320 starts the timer with duration T for delayingnotification of the first handover to the gateway 350. Further, thecontrol node 320 stores information identifying the first BS 110 as thefirst source BS of a potential handover sequence. By message 614, thecontrol node 320 acknowledges the path switch request of message 613.

Upon receiving message 614, the second BS 120 initiates releasing of UEresources at the source BS 110 by sending message 615 to the first BS110. Message 615 may be UE context release message in accordance with3GPP TS 23.401 and 36.300.

Upon receiving message 615, the first BS 110 releases resources relatedto the UE 10, as indicated by step 616. However, the first BS 110 keepssufficient state information to continue forwarding downlink user planedata of the UE 10. In accordance with the illustrated concepts, thefirst BS 110 is configured to continue with packet forwarding for theforwarding time period T_(f) which is longer than the delay time periodT for delaying the control signalling to the gateway. For this purpose,a forwarding timer of duration T_(f) may be started together with theforwarding of user plane data by the source BS 110, i.e., upon sendingmessage 606.

The second handover from the second BS 120 back to the first BS 110 isthen prepared by messages 617 and 618. Message 617 may be a handoverrequest from the second BS 120 to the first BS 110, and message 618 maybe a handover request acknowledgement form the first BS 110 to thesecond BS 120, in accordance with 3GPP TS 36.300.

By sending message 619 to the UE 10, the second BS 120 then initiatesexecution of the second handover. In accordance with 3GPP TS 36.300,message 619 may be a radio resource control connection reconfigurationmessage and convey mobility control information to the UE 10. Asillustrated by step 620, the UE 10 may then detach from the cell of thesecond BS 120 and synchronize to the cell of the first BS 110. Thesecond BS 120 may then send status information to the first BS 110,using message 621.

As illustrated by message 622, the second BS 120 may then startforwarding downlink user plane data to the first BS 110. Here, it is tobe understood that it is the first BS 110 which in turn forwards suchdownlink user plane data to the second BS 120.

By sending message 623 to the first BS 110, the UE 10 then again sets upa connection to the first BS 110. After this, uplink user plane data canbe conveyed directly via the first BS 110 to the gateway 350, asillustrated by messages 624 and 625, while downlink user plane data canbe conveyed from the gateway 350 to the first BS 110 and then forwardedvia the first BS 110, the second BS 120, and then again the first BS 110to the UE 10, as illustrated by messages 626 to 629.

By sending message 630 to the control node 320, the first BS 110 thenrequests path switching to the first data path P1. The message 630 maybe a path switch request in accordance with 3GPP TS 23.401 and 36.300.In the scenario of FIG. 6, it is assumed that the timer started uponnotification of the first handover by message 613 has not yet expired atthis point. Accordingly, the control node 320 skips notifying the firsthandover to the gateway 350. Further, on the basis of the informationidentifying the first BS 110 as the first source BS of the handoversequence, the control node 320 also skips notifying the second handoverto the gateway 350, irrespective of expiration of the delay time periodT starting at notification of the second handover to the control node320.

Upon receiving message 631, the first BS 110 initiates releasing of UEresources at the second BS 120 by sending message 632 to the second BS120. Message 632 may be UE context release message in accordance with3GPP TS 23.401 and 36.300.

Upon receiving message 632, the second BS 120 releases resources relatedto the UE 10, as indicated by step 633. However, the second BS 120 keepssufficient state information to continue forwarding downlink user planedata of the UE 10. In accordance with the illustrated concepts, thesecond BS 120 is configured to continue with packet forwarding for theforwarding time period T_(f) which is longer than the delay time periodT for delaying the control signalling to the gateway. For this purpose,a forwarding timer of duration T_(f) may be started together with theforwarding of user plane data by the second BS 120, i.e., upon sendingmessage 622.

At step 634, the first BS 110 stops forwarding of user plane data. Thismay be accomplished upon expiry of the forwarding timer of durationT_(f). Having stopped forwarding user plane data, the first BS 110 mayalso release the resources associated with the forwarding of user planedata. The downlink user plane data may then be forwarded from thegateway 350 to the first BS 110 and then directly to the UE 10, asindicated by messages 635 and 636. At step 637, also the second BS 120stops forwarding of user plane data. This may be accomplished uponexpiry of the forwarding timer of duration T_(f). Having stoppedforwarding user plane data, the second BS 120 may also release theresources associated with the forwarding of user plane data.

As can be seen, in the scenario of FIG. 6, forwarding of user plane datafrom the first BS 110 to the second BS 120 and then back to the first BS110 may occur. In view of efficient resource usage and delay, it isdesirable to avoid such forwarding loops, in particular when consideringthat such forwarding loops may be more complex than in the ping-pongscenario of FIG. 6, for example a handover sequence may include a firsthandover from a first BS to a second BS, a second handover from thesecond BS to a third BS, a third handover from the third BS to a fourthBS, and a fourth handover from the fourth BS back to the second BS.

For avoiding forwarding loops, a BS to which the UE 10 is presentlyconnected may store an identifier of the UE 10 when there is noforwarding of user plane data of the UE 10 from this BS. Such identifiermay be stored in a UE context and forwarded during handover to anotherBS. The identifier could for example be based on or an Internet Protocoladdress of the gateway 350 and a Tunnel Endpoint Identifier (TEID) foruplink user plane data traffic.

Further, the identifier could be based on a cell identifier of thepresent cell concatenated by a UE identifier within this cell, e.g., bya Cell Radio Network Temporary Identity (C-RNTI). The identifier couldbe stored as long as forwarding of user plane data takes place. By usingthe identifier, the BS may detect that the UE 10 returns to the BS whilepacket forwarding takes place, since the UE has the same identifier inthe UE context as another context related to packet forwarding. If sucha situation is detected by the BS, it can stop forwarding of the userplane data.

For avoiding forwarding loops, it is also possible that the control node320 detects that the UE 10 returns to an earlier BS of a sequence ofhandovers. This could be based on the identity of the first BS of asequence of handovers. For example such identity could be stored in a UEcontext of the control node 320. The control node 320 may also store theidentities of all BSs of the sequence of handovers since forwarding hasstarted, and detect when the UE 10 returns to one of the earlier BSs.When returning to an earlier BS of the sequence of handovers isdetected, the control node can indicate this to the BS, e.g., by a flagin the path switch request acknowledgement message. Based on thisindication, the BS can stop forwarding of the user plane data, therebyremoving the forwarding loop.

In the above concepts, a number of criteria may be used to control thedelay of the handover notification from the control node 320 to thegateway 350 and the extended forwarding of user plane data.

The basic criterion is the delay time period T from the notification ofthe last handover to the control node. By delaying the notification ofthe gateway 350 until this delay time period T expires, a sequence ofmultiple handovers may cause only a single notification to the gateway350. As a further criterion, a maximum number P_(max) of handoversnotified to the control node 320 may be defined, e.g., by implementing acorresponding counter. If the number of handovers notified to thecontrol node 320 reaches this maximum number, the gateway 350 may benotified even if the delay time period T has not yet expired. In thisway, the number of hops for forwarding user plane data can be limited.

As a further criterion, a timeout T_(t) for the total time since thenotification of the first handover of a sequence to the control node 320may be used, e.g., by implementing a further timer. Upon reaching thetimeout T_(t), the gateway 350 may be notified even if the delay timeperiod T has not yet expired. Here, the first handover of the sequencemay be defined as the first handover after last notifying the gateway350 of a handover. In this way, the notification of the gateway 350 isnot delayed more than T_(t) even if there are multiple handovers withinthe delay time period T.

If the delay time period T is used in connection with the maximum numberof notified handovers P_(max), the forwarding time period T_(f) can beset to be at least T_(f)=T×P_(max). If the delay time period T is usedin connection with the timeout T_(t), the forwarding time period T_(f)can be set to be at least T_(f)=T_(t). The values for T, P_(max), andT_(t) may differ between different BSs of the mobile network. Thesevalues may be statically configured in the control node 320, e.g., perBS or per group of BSs, per Tracking Area, on the basis of geographicallocation, or the like. Further, these values could be dynamicallyobtained either through provisioning over an external interface orthrough a self learning system in the control node 320, thereby allowingfor an effective balancing between on the one hand handover relatedsignalling load and on the other hand forwarding related load on theinter-BS interfaces.

In some scenarios, it is also useful to restart the timer or timersand/or the counter of notified handovers when the UE 10 to an earlier BSof a sequence of handovers, i.e., when a forwarding loop is detected. Inthis way the signalling reduction in the case of ping-pong or loopbehaviour can be further improved.

As mentioned above, the delay of the handover notification may also beimplemented at the target BS. Corresponding scenarios will be furtherexplained in the following.

Similar to the above scenarios, implementing the delay at the target BSmay utilize forwarding of user plane data between the source BS and thetarget BS until the gateway 350 is finally notified. However, in thiscase notification of the handover from the target BS to the control nodeis delayed by notifying the control node after expiry of the delay timeperiod T. For this purpose a corresponding timer in the target BS isused. In this way the handover related signalling load between the RAN100 and the control node 320 and between the control node 320 and thegateway 350 can be reduced.

When implementing the delay at the target BS, a situation may occur inwhich control plane data need to be transmitted between the target BSand the control node 320 after execution of the handover, but beforenotification of the handover to the control node. Such a situation maybe addressed by forwarding the control plane data between the source BSand the target BS. In the scenarios as illustrated in FIGS. 1, 3, and 4,this forwarding of control plane data may be implemented by routing apart of the S1-MME connection between the target BS and the control node320 over the X2 inter-BS interface. Alternatively, the need fortransmission of control plane data may also trigger the notification ofthe handover to the control node 320 so that the control plane data canbe transmitted directly between the target BS and the control node 320,using an interface provided between the target BS and the control node,such as the S1-MME interface illustrated in FIGS. 1, 3, and 4.

FIG. 7 shows a timing diagram for illustrating procedures in a handoverscenario as illustrated in FIG. 1. The procedures involve the UE 10, thesource BS 110, the target BS 120, the control node 320, and the gateway350. In the scenario of FIG. 2, notifying the gateway 350 of a handoverof the UE 10 from the source BS 110 to the target BS 120 is delayed atthe target BS 120, and forwarding of user plane data between the sourceBS 110 and the target BS 120 takes place until the gateway 350 isnotified of the handover and switches to the second data path P2established using the interface between the target BS 120 and thegateway 350. Further, also forwarding of control plane data between thesource BS 110 and the target BS 120 is possible before the control node320 is notified of the handover.

In the procedures of FIG. 7, the handover from the source BS 110 to thetarget BS 120 is prepared by messages 701 and 702. Message 701 may be ahandover request from the source BS 110 to the target BS 120, andmessage 702 may be a handover request acknowledgement form the target BS120 to the source BS 110, in accordance with 3GPP TS 36.300.

By sending message 703 to the UE 10, the source BS 110 then initiatesexecution of the handover. In accordance with 3GPP TS 36.300, message703 may be a RRC connection reconfiguration message and convey mobilitycontrol information to the UE 10. As illustrated by step 704, the UE 10may then detach from the cell of the source BS 110 and synchronize tothe cell of the target BS 120. The source BS 110 may then send statusinformation to the target BS 120, using message 705.

As illustrated by message 705, the source BS 110 may then startforwarding downlink user plane data to the target BS 120.

By sending message 707 to the target BS 120, the UE 10 then sets up aconnection to the target BS 120. After this, uplink user plane data canbe conveyed directly via the target BS 120 to the gateway 350, asillustrated by messages 708 and 709, while downlink user plane data canbe conveyed from the gateway 350 to the source BS 110 and then forwardedvia the target BS 120 to the UE 10, as illustrated by messages 710 to712.

In the procedures of FIG. 7, execution of the handover is completed bythe target BS 120 sending message 713 to the source BS 110, to initiatereleasing of UE resources at the source BS 110. Message 713 may be UEcontext release message in accordance with 3GPP TS 23.401 and 36.300.Further, the target BS 120 starts a timer having a duration whichcorresponds to the delay time period T for delaying notification of thehandover to the control node 320. The duration of the timer, i.e., thedelay time period T, may be configurable, e.g., through OAM procedures.

Upon receiving message 713, the source BS 110 releases resources relatedto the UE 10, as indicated by step 714. However, the source BS 110 keepssufficient state information to continue forwarding downlink user planedata of the UE 10. In accordance with the illustrated concepts, thesource BS 110 is configured to continue with packet forwarding for aforwarding time period T_(f) which is longer than the delay time periodT for delaying the control signalling to the control node 320. For thispurpose, a forwarding timer of duration T_(f) may be started togetherwith the forwarding of user plane data by the source BS 110, i.e., uponsending message 706. The forwarding time period T_(f) may beconfigurable, e.g., through OAM procedures. Alternatively oradditionally, the use of the longer forwarding time period T_(f) may benegotiated by the target BS 120 and the BS 110 during the setup of theinter-BS connection therebetween. It may also be possible for the targetBS 120 to store information indicating which BS supports a sufficientlylong forwarding time period T_(f) and to apply the delaying of thehandover notification to the control node 320 only if the source BS 110supports such a sufficiently long forwarding time period T_(f). Thetarget BS 120 may learn such information from the other BSs and/or maybe configured with such information in advance, e.g., on a per TAIbasis, or on a per network domain network domain basis. Further, thesource BS 110 maintains context information allowing for continuedsignalling with the target BS 120 over the inter-BS interface, inparticular for forwarding control plane data between the source BS 110and the target BS 120.

At step 715, the timer expires. In the scenario of FIG. 7, it is assumedthat the timer expires before another handover of the UE 10. Afterexpiry of the timer, the target BS 120 requests path switching to thesecond data path P2 established using the interface between the targetBS 120 and the gateway 350. This is accomplished by sending message 716to the control node 320. The message 716 may be a path switch request inaccordance with 3GPP TS 23.401 and 36.300.

In response to the path switch request of message 716, the control node320 notifies the gateway 350 of the handover. This is accomplished bysending message 717 to the gateway 350. Message 717 may be a ModifyBearer Request message in accordance with 3GPP TS 23.401. Upon receivingthe message 717, the gateway 350 switches conveying of user plane datato the second data path P2 of FIG. 1. Accordingly, as illustrated bymessages 718 and 719, downlink user plane data are no longer conveyedvia the source BS 110.

For facilitating packet reordering, the gateway 350 may then send an endmarker via the source BS 110 to the target BS 120, as illustrated bymessages 720 and 721. The sending of the end marker may be accomplishedin accordance with 3GPP TS 23.401 and 36.300.

By sending message 722 to the control node 320, the gateway 350 thenresponds to the notification of message 717. Message 722 may be a ModifyBearer Response accordance with 3GPP TS 23.401.

By message 723, the control node 320 acknowledges the path switchrequest of message 716.

At step 724, the source BS 110 may release all resources associated withthe UE 10, including the resources for forwarding of user plane data andthe context for signalling with the target BS 120. This may beaccomplished in response to signalling from the target BS 120 or uponexpiry of a timer, e.g., the forwarding timer of duration T_(f).

Also when delaying the signalling of a handover at the target BS, therecan be a sequence of multiple handovers within the delay time period.The forwarding of user plane data and/or of control plane data betweenthe BSs involved in the sequence of handovers can then be accomplishedby chaining as explained in connection with FIG. 3, i.e., by forwardingthe user plane data and/or control plane data between adjacent BSs ofthe handover sequence, or by anchoring as explained in connection withFIG. 4, e.g., by forwarding the user plane data and/or control planedata directly between the source BS of the first handover of thesequence and the target BS of the last handover of the sequence, using acorresponding inter-BS interface therebetween. Also in this case, theanchoring option may provide a more efficient usage of inter-BSinterface resources.

FIG. 8 shows a timing diagram for illustrating procedures in a handoverscenario as illustrated in FIG. 3. The procedures involve the UE 10, thefirst BS 110, the second BS 120, the third BS 130, the control node 320,and the gateway 350. A sequence of a first handover from the first BS110 to the second BS 120 and a second handover from the second BS 120 tothe third BS 130 is assumed. In the scenario of FIG. 8, notifying thecontrol node 320 of the first handover and the second handover isdelayed at the respective target BSs, i.e., notification of the firsthandover to the control node 320 is delayed at the second BS 120, and iteventually does not take place as the second handover comes earlier.Notification of the second handover to the control node 320 is delayedat the third BS 130. Forwarding of downlink user plane data from thefirst BS 110 to the third BS 130 is accomplished via the second BS 120and takes place until the gateway 350 is notified of the second handoverand switches to the third data path P3 established using the interfacebetween the third BS 130 and the gateway 350. Further, also forwardingof control plane data between the first BS 110, the second BS 120, andthe third BS 130 is possible before the control node 320 is notified ofthe second handover.

At step 801, the first handover from the first BS 110 to the second BS120 is prepared and executed. This may be accomplished as explained inconnection with messages and steps 701 to 714 of FIG. 7. However, ascompared to FIG. 7 it is now assumed that the second handover from thesecond BS 120 to the third BS 130 is executed before expiry of the timerstarted when completing execution of the first handover.

In the procedures of FIG. 8, the handover from the second BS 120 to thethird BS 130 is prepared by messages 802 and 803. Message 802 may be ahandover request from the second BS 120 to the third BS 130, and message803 may be a handover request acknowledgement form the third BS 130 tothe second BS 120, in accordance with 3GPP TS 36.300.

By sending message 804 to the UE 10, the second BS 120 then initiatesexecution of the second handover. In accordance with 3GPP TS 36.300,message 804 may be a RRC connection reconfiguration message and conveymobility control information to the UE 10. As illustrated by step 805,the UE 10 may then detach from the cell of the second BS 120 andsynchronize to the cell of the third BS 130. The second BS 120 may thensend status information to the third BS 130, using message 806.

As illustrated by message 807, the second BS 120 may then startforwarding downlink user plane data to the third BS 130. Here, it is tobe understood that it is the first BS 110 which in turn forwards suchdownlink user plane data to the second BS 120.

By sending message 808 to the third BS 130, the UE 10 then sets up aconnection to the third BS 130. After this, uplink user plane data canbe conveyed directly via the third BS 130 to the gateway 350, asillustrated by messages 809 and 810, while downlink user plane data canbe conveyed from the gateway 350 to the first BS 110 and then forwardedvia the second BS 120 and the third BS 130 to the UE 10, as illustratedby messages 811 to 814.

In the procedures of FIG. 8, execution of the second handover iscompleted by the third BS 130 sending message 815 to the second BS 120,to initiate releasing of UE resources at the second BS 120. Message 815may be UE context release message in accordance with 3GPP TS 23.401 and36.300. Further, the third BS 130 starts a timer having a duration whichcorresponds to the delay time period T for delaying notification of thehandover to the control node 320. The duration of the timer, i.e., thedelay time period T, may be configurable, e.g., through OAM procedures.

Upon receiving message 815, the second BS 120 releases resources relatedto the UE 10, as indicated by step 816. However, the second BS 120 keepssufficient state information to continue forwarding downlink user planedata of the UE 10. In accordance with the illustrated concepts, thesecond BS 120 is configured to continue with packet forwarding for aforwarding time period T_(f) which is longer than the delay time periodT for delaying the control signalling to the control node 320. For thispurpose, a forwarding timer of duration T_(f) may be started togetherwith the forwarding of user plane data by the second BS 120, i.e., uponsending message 806. The forwarding time period T_(f) may be configuredin a similar manner as explained in connection with step 714 of FIG. 7.Further, the second BS 120 maintains context information allowing forcontinued signalling with the third BS 130 over the inter-BS interface,in particular for forwarding control plane data between the second BS120 and the third BS 130.

At step 817, the timer expires. In the scenario of FIG. 8, it is assumedthat the timer expires before another handover of the UE 10. Afterexpiry of the timer, the third BS 130 requests path switching to thethird data path P3 established using the interface between the third BS130 and the gateway 350. This is accomplished by sending message 818 tothe control node 320. The message 818 may be a path switch request inaccordance with 3GPP TS 23.401 and 36.300.

In response to the path switch request of message 818, the control node320 notifies the gateway 350 of the second handover. This isaccomplished by sending message 819 to the gateway 350. Message 819 maybe a Modify Bearer Request message in accordance with 3GPP TS 23.401.Upon receiving the message 819, the gateway 350 switches conveying ofuser plane data to the third data path P3 of FIG. 3. Accordingly, asillustrated by messages 820 and 821, downlink user plane data are nolonger conveyed via the first BS 110 and the second BS 120.

For facilitating packet reordering, the gateway 350 may then send an endmarker via the first BS 110 and the second BS 120 to the third BS 130,as illustrated by messages 822 to 824. The sending of the end marker maybe accomplished in accordance with 3GPP TS 23.401 and 36.300.

By sending message 825 to the control node 320, the gateway 350 thenresponds to the notification of message 819. Message 825 may be a ModifyBearer Response accordance with 3GPP TS 23.401.

By message 826, the control node 320 acknowledges the path switchrequest of message 818.

At step 827, the second BS 120 may release all resources associated withthe UE 10, including the resources for forwarding of user plane data andthe context for signalling with the third BS 130. This may beaccomplished in response to signalling from the third BS 130 or uponexpiry of a timer, e.g., the forwarding timer of duration T_(f).Similarly, at step 828, the first BS 110 may release all resourcesassociated with the UE 10, including the resources for forwarding ofuser plane data and the context for signalling with the third BS 120.This may be accomplished in response to signalling from the second BS120 or upon expiry of the forwarding timer of duration T_(f).

In the example of FIG. 8, the handover related signalling between theRAN 100 and the control node 320, and also between the control node 320and the gateway 350 is significantly reduced, because the first handoveris not notified to the control node 320, and consequently not to thegateway 350. Instead, only the second handover, which followed the firsthandover within the delay time period T, is notified.

As mentioned above, when delaying the notification of the handover fromthe target BS to the control node 320, control plane data exchanged bythe target BS and the control node 320 may be via other BSs in thehandover sequence. The forwarded control plane data may for exampleinclude Non Access Stratum (NAS) messages of the S1-MME interface orother messages of the S1-MME interface without NAS content. Thenotification itself, in the examples of FIG. 7 and the path switchrequest of messages 716 and 818, may however be sent directly from thetarget BS to the control node 320.

Forwarding the control plane data between two BSs may be accomplishedusing an existing control plane association between the BSs, e.g., onthe basis of the X2 inter-base station interface. For this purpose, anew type of control message may be defined or an existing controlmessage may be modified. In some scenarios, an identifier for the X2control plane between to BSs can also be used to identify the UE 10.When forwarding, a BS may replace an S1/X2 identifier with a UEidentifier at the target BS or with an X2/S1 identifier of the controlnode 320. In this way, it is possible to re-use the UE identifiers ofestablished X2 connections and it is not necessary to forward an S1/X2specific identifiers on another interface. Further, a special containermessage format for the X2 interface may be defined for the purpose ofcontrol message forwarding. The forwarding of the control plane data maybe applied to both uplink control plane data transmitted from the targetBS to the control node 320 and to downlink control plane datatransmitted from the control node 320 to the target BS. In somescenarios, control plane data to be forwarded may be temporarilybuffered by a BS, e.g., if the UE 10 has already moved to a new BS, butthere has not yet been any signalling from the new BSs.

A timing diagram for illustrating corresponding forwarding procedures isillustrated in FIG. 9. The procedures of FIG. 9 involve the first BS110, the second BS 120, the third BS 130, and the control node 320,e.g., as in the scenarios of FIGS. 3 and 8.

In the timing diagram of FIG. 9, it is first assumed that the third BS130 needs to send uplink control plane data to the control node 320. Thecontrol plane data may for example include a control message of theS1-MME interface.

By message 901, the third BS 130 forwards the control plane data to thesecond BS 120. The message 901 is be transmitted over the inter-BSinterface between the third BS 130 and the second BS 120. By message902, the second BS 120 then forwards the control plane data to the firstBS 110. The message 902 is transmitted over the inter-BS interfacebetween the second BS 120 and the first BS 110.

By message 903, the first BS 110 forwards the control plane data to thecontrol node, using the direct control plane interface between the firstBS 110 and the control node 320.

Next it is assumed that the control node 320 needs to send downlinkcontrol plane data. Again, the control plane data may for exampleinclude a control message of the S1-MME interface. However, due to thedelayed notification of handovers, the control node 320 may be not awarethat the UE 10 executed handovers to the second BS 120 and then to thethird BS 130. Accordingly, by message 911 the control node 320 sends thecontrol plane data to the first BS 110.

The first BS 110, which is aware of the handover to the second BS 120,then forwards the control plane data to the second BS 120, using message912. The message 912 is transmitted over the inter-BS interface betweenthe first BS 110 and the second BS 120. The second BS 120, which in turnis aware of the handover to the third BS 130, then forwards the controlplane data to the third BS 130, using message 913. The message 913 istransmitted over the inter-BS interface between the second BS 120 andthe third BS 130.

As further mentioned above, it is also possible that the need to sendcontrol plane data triggers notification of the handover to the controlnode 320, irrespective of the expiry of the delay time period. Anexample of corresponding procedures for uplink control plane data isillustrated in FIG. 10, and an example of corresponding procedures fordownlink control plane data is illustrated in FIG. 11.

In the procedures of FIG. 10, it is assumed that the third BS 130 needsto send uplink control plane data to the control node 320. The controlplane data may for example include a control message of the S1-MMEinterface. However, due to the delaying of the notification of thehandover to the control node 320, the control node 320 is not aware ofthe handover of the UE 10 to the third BS 130. Accordingly, the third BS130 may buffer the control plane data, notify the control node 320 ofthe handover by sending the path switch request, and then send thecontrol plane data directly to the control node 320.

The procedures of FIG. 10 involve the UE 10, the first BS 110, thesecond BS 120, the third BS 130, the control node 320, and the gateway350. In the initial situation underlying the procedures of FIG. 10, itis assumed that the UE 10 performed a first handover from the first BS110 to the second BS 120 and then a second handover from the second BS120 to the third BS 130, so that the UE 10 is connected to the third BS130, but the core network, in particular the control node 320, is stillconnected to the first BS 110, as illustrated by step 1001. In thissituation, uplink user plane data can be conveyed directly via the thirdBS 130 to the gateway 350, as illustrated by messages 1002 and 1003,while downlink user plane data can be conveyed from the gateway 350 tothe first BS 110 and then forwarded via the second BS 120 and the thirdBS 130 to the UE 10, as illustrated by messages 1004 to 1007.

In this situation, the third BS 130 may have a need to send uplinkcontrol plane data to the control node 320, e.g., a control message ofthe S1-MME interface. For example, such need may be caused by the UE 10sending message 1008 to the third BS 130. The message 1008 may be a NASmessage.

At step 1009, the need to send the uplink control data arises at thethird BS 130. In response to detecting this need, the third BS 130notifies the control node 320 of the handover to the third BS 130, toupdate the user and control plane data paths. This is accomplished bysending message 1010 to the control node 320. Message 1010 may be a pathswitch request in accordance with 3GPP TS 23.401 and 36.300. In responseto message 1010, the control node 320 notifies the gateway 350 of thehandover. This is accomplished by sending message 1011 to the gateway350. Message 1011 may be a Modify Bearer Request message in accordancewith 3GPP TS 23.401. Upon receiving the message 1011, the gateway 350switches conveying of user plane data to the data path P3 establishedvia the interface between the third BS 130 and the gateway 350. Bysending message 1012 to the control node 320, the gateway 350 thenresponds to the notification of message 1011. Message 1012 may be aModify Bearer Response accordance with 3GPP TS 23.401. By sendingmessage 1013 to the third BS 130, the control node 320 acknowledges thepath switch request of message 1010.

Message 1012 confirms to the third BS 130 that the control node 320 nowis aware of the UE 10 being connected to the third BS 130. Accordingly,the third BS 130 may then proceed by sending the control plane datadirectly to the control node 320, using message 1014 which istransmitted via the interface provided between the third BS 130 and thecontrol node 320.

At step 1015, the second BS 120 may release all resources associatedwith the UE 10, including the resources for forwarding of user planedata and the context for signalling with the third BS 130. Similarly, atstep 1016, the first BS 110 may release all resources associated withthe UE 10, including the resources for forwarding of user plane data andthe context for signalling with the third BS 120.

In the procedures of FIG. 11, it is assumed that the control node 320needs to send downlink control plane data to the BS to which the UE 10is connected. The control plane data may for example include a controlmessage of the S1-MME interface.

The procedures of FIG. 11 involve the UE 10, the first BS 110, thesecond BS 120, the third BS 130, the control node 320, and the gateway350. In the initial situation underlying the procedures of FIG. 11, itis assumed that the UE 10 performed a first handover from the first BS110 to the second BS 120 and then a second handover from the second BS120 to the third BS 130, so that the UE 10 is connected to the third BS130, but the core network, in particular the control node 320, is stillconnected to the first BS 110, as illustrated by step 1001. In thissituation, uplink user plane data can be conveyed directly via the thirdBS 130 to the gateway 350, as illustrated by messages 1102 and 1103,while downlink user plane data can be conveyed from the gateway 350 tothe first BS 110 and then forwarded via the second BS 120 and the thirdBS 130 to the UE 10, as illustrated by messages 1104 to 1107.

In this situation, if the need for sending downlink control plane dataarises as illustrated at step 1108. The delaying of the notification ofthe handover to the control node 320 causes the control node 320 to benot aware of the handover of the UE 10 to the third BS 130. Rather, thecontrol node 320 assumes that the UE 10 is still connected to the firstBS 110. Accordingly, the control node 320 attempts to send the controlplane data by message 1109 to the first BS 110. However, because the UE10 is no longer connected to the first BS 110, the first BS 1110indicates a failure by sending message 1110 to the control node 320.Message 1110 may for example include a failure cause, i.e., that ahandover was performed, such as defined in 3GPP TS 23.401.

In response to receiving the message 1110, the control node 320 mayarrange for a retransmission of the control plane data after a certainretransmission delay period. For this purpose, a corresponding timer maybe started. The retransmission delay period may be configurable, e.g.,through OAM procedures. If the need to transmit the control plane datawas originally triggered by a control message from the gateway 350, thecontrol node 320 may also provide a failure indication to the gateway350, e.g., by returning an error cause such as “Temporarily rejected dueto handover procedure in progress” as defined in 3GPP TS 29.274. Thismay in turn cause retransmission of the control message by the gateway350.

In response to receiving message 1109, the first BS 110 also sendsmessage 1112 to the BS to which the UE 10 was handed over from the firstBS 110. In the present case, this would be the second BS 120. Message1112 includes a command for controlling the BS to which the UE 10 isconnected to initiate path switching by notifying the control node 320of the last handover of the UE 10. The command is forwarded by the BSsuntil the target BS of the last handover in the sequence is reached.Accordingly, in the illustrated example the second BS 120 sends message1113 to the third BS 130 and thereby forwards the command to the thirdBS 130 to which the UE 10 is connected.

In response to receiving message 1113, the third BS 130 executes thecommand by notifying the control node 320 of the handover, to update theuser and control plane data paths. This is accomplished by sendingmessage 1114 to the control node 320. Message 1114 may be a path switchrequest in accordance with 3GPP TS 23.401 and 36.300.

In response to message 1114, the control node 320 notifies the gateway350 of the handover. This is accomplished by sending message 1115 to thegateway 350. Message 1115 may be a Modify Bearer Request message inaccordance with 3GPP TS 23.401. Upon receiving the message 1115, thegateway 350 switches conveying of user plane data to the data path P3established via the interface between the third BS 130 and the gateway350. By sending message 1116 to the control node 320, the gateway 350then responds to the notification of message 1115. Message 1116 may be aModify Bearer Response accordance with 3GPP TS 23.401. By sendingmessage 1117 to the third BS 130, the control node 320 acknowledges thepath switch request of message 1114.

The control node 320, which is now aware of the UE 10 being connected tothe third BS 130, may then proceed by retransmitting the control planedata directly to the third BS 130, using message 1118 which istransmitted via the interface provided between the third BS 130 and thecontrol node 320. This may be accomplished after expiry of theabove-mentioned retransmission delay period, e.g., as indicated by theexpiry of the corresponding timer.

At step 1119, the second BS 120 may release all resources associatedwith the UE 10, including the resources for forwarding of user planedata and the context for signalling with the third BS 130. Similarly, atstep 1120, the first BS 110 may release all resources associated withthe UE 10, including the resources for forwarding of user plane data andthe context for signalling with the third BS 120.

FIG. 12 shows a timing diagram for illustrating procedures for avoidingforwarding loops in handover sequences while using delaying of thehandover notification at the target BS. The procedures involve the UE10, the first BS 110, the second BS 120, the third BS, the control node320, and the gateway 350, as illustrated in FIG. 3. As compared to FIG.3, an extended sequence of handovers is assumed: a first handover of theUE 10 from the first BS 110 to the second BS 120, a second handover ofthe UE 10 from the second BS 120 to the third BS 130, and a thirdhandover of the UE 10 from the third BS 130 back to the second BS 120.With respect to the first handover, the first BS 110 is the source BSand the second BS 120 is the target BS. With respect to the secondhandover, the second BS 120 is the source BS and the third BS 130 is thetarget BS. With respect to the third handover, the third BS 130 is thesource BS and the second BS 120 is the target BS. In the scenario ofFIG. 12, notifying the gateway 350 of the handovers is delayed at theBSs 110, 120, 130 while forwarding of user plane data is accomplishedbetween the first BS 110, the second BS 120, and the third BS 130.Further, also forwarding of control plane data between the first BS 110,the second BS 120, and the third BS 130 is possible.

At step 1201, the first handover from the first BS 110 to the second BS120 is prepared and executed. This may be accomplished as explained inconnection with messages and steps 701 to 714 of FIG. 7. However, inthis case it is further assumed that the request for the first handoverfrom the first BS 110 to the second BS 120 includes an anchoridentifier. The anchor identifier uniquely identifies the UE 10 and thefirst source BS of the sequence of handovers, and is forwarded to thenext BS of the handover sequence in the respective handover request. Theanchor identifier can be the concatenation of an IP address of the firstsource BS of the sequence and an identifier of the UE as used inmessages of the inter-BS interface. Other identifiers could be used aswell, such as an uplink IP address of the gateway 350 and the TEID. Ascompared to FIGS. 7 and 8 it is further assumed that the secondhandover, which is prepared and executed at step 1202, is executedbefore expiry of the timer started when completing execution of thefirst handover and also that the third handover is executed beforeexpiry of the timer started when completing execution of the secondhandover. Accordingly, the second BS 120 may skip notification of thefirst handover to the control node 320, and the third BS 130 may skipnotification of the second handover to the control node 320.

In the procedures of FIG. 12, the third handover from the third BS 130back to the second BS 120 is prepared by messages 1203 and 1204. Message1203 may be a handover request from the first BS 130 to the second BS120, and message 1204 may be a handover request acknowledgement form thesecond BS 120 to the third BS 130, in accordance with 3GPP TS 36.300.Message 1204 includes the anchor identifier which was forwarded from thesecond BS 120 to the third BS 130 in the request for the secondhandover.

By sending message 1205 to the UE 10, the third BS 130 then initiatesexecution of the third handover. In accordance with 3GPP TS 36.300,message 1205 may be a RRC connection reconfiguration message and conveymobility control information to the UE 10. As illustrated by step 1206,the UE 10 may then detach from the cell of the third BS 130 andsynchronize to the cell of the second BS 120. The third BS 130 may thensend status information to the second BS 120, using message 1207.

As illustrated by message 1208, the third BS 130 may then startforwarding downlink user plane data to the second BS 120.

By sending message 1209 to the second BS 120, the UE 10 then sets up aconnection to the second BS 120. After this, uplink user plane data canbe conveyed directly via the second BS 120 to the gateway 350, asillustrated by messages 1210 and 1211, while downlink user plane datacan be conveyed from the gateway 350 to the first BS 110 and thenforwarded via the second BS 120, the third BS 130, and then again thesecond BS 120 to the UE 10, as illustrated by messages 1212 to 1217.

In the procedures of FIG. 12, execution of the third handover iscompleted by the second BS 120 sending message 1218 to the third BS 130,to initiate releasing of UE resources at the second BS 130. Message 1218may be UE context release message in accordance with 3GPP TS 23.401 and36.300. Further, the second BS 120 restarts the timer with having aduration T for delaying notification of the handover to the control node320.

Upon receiving message 1218, the third BS 130 releases resources relatedto the UE 10, as indicated by step 1219. However, the third second BS130 keeps sufficient state information to continue forwarding downlinkuser plane data of the UE 10. In accordance with the illustratedconcepts, the third second BS 130 is configured to continue with packetforwarding for a forwarding time period T_(f) which is longer than thedelay time period T for delaying the notification of the handover to thecontrol node 320. For this purpose, a forwarding timer of duration T_(f)may be started together with the forwarding of user plane data by thesecond BS 120, i.e., upon sending message 1208. The forwarding timeperiod T_(f) may be configured in a similar manner as explained inconnection with step 714 of FIG. 7. Further, the third BS 130 maintainscontext information allowing for continued signalling with the second BS120 over the inter-BS interface, in particular for forwarding controlplane data between the third BS 130 and the second BS 120.

As can be seen from messages 1215 and 1216, a forwarding loop is formedby the second BS 120 and the third BS 130. The second BS 120 detectsthis forwarding loop at step 1220. This is accomplished on the basis ofthe anchor identifier received from the third BS 130 in message 1203. Inparticular, the second BS 120 may determine that the anchor identifieris the same as another anchor identifier associated with trafficforwarding by the second BS 120, namely the anchor identifier receivedfrom the first BS 110 together with the request for the first handover.

For removing the forwarding loop, the second BS 120 sends message 1221to the previous BS in the sequence of handovers, i.e., to the third BS130. The message 1221 includes a loop removal command. The loop removalcommand is forwarded from one BS of a sequence of handovers to theprevious one, i.e., to the adjacent source BS, until it has travelledthrough the loop, i.e., returned to the BS which generated the loopremoval command. In the scenario of FIG. 12, this means that the loopremoval command is forwarded from the third BS 130 to the second BS 120,but not further. In each BS receiving the loop removal command,forwarding can be stopped and the corresponding resources and contextreleased. In the scenario of FIG. 12, this means that the second BS 120stops forwarding with respect to the third BS 130 and releasescorresponding resources and context, as illustrated at step 1223, andthat the third BS 130 stops forwarding with respect to the second BS 120and releases corresponding resources and context, as illustrated at step1224. In this way, the forwarding loop is removed. The stopping offorwarding and releasing of the corresponding context and resources canbe delayed by a configurable time interval to account for delays in theforwarded user plane and/or control plane data and to allow forreordering of data packets.

Having removed the forwarding loop, downlink user plane data can beconveyed from the gateway 350 to the first BS 110 and then forwarded viathe second BS 120 directly to the UE 10 as illustrated by messages 1227to 1229.

FIG. 13 shows a timing diagram for illustrating procedures for handlingof a ping-pong scenario while delaying notification of the handover atthe target BS. In the scenario of FIG. 13, it is assumed that due tocertain restrictions, forwarding loops may not exceed two hops. Thiscould be accomplished by imposing a limit of two on the maximum numberof handovers in a sequence before notification to the control node 320,e.g., by forwarding an indication of the number of handovers in sequencebetween the BSs of the sequence. The procedures involve the UE 10, thefirst BS 110, the second BS 120, the control node 320, and the gateway350, as illustrated in FIG. 1. In the scenario of FIG. 13, a sequence ofhandovers is assumed which includes a first handover of the UE 10 fromthe first BS 110 to the second BS 120, and a second handover of the UE10 from the second BS 120 back to the first BS 110. With respect to thefirst handover, the first BS 110 is the source BS and the second BS 120is the target BS. With respect to the second handover, the second BS 120is the source BS and the first BS 110 is the target BS. In the scenarioof FIG. 13, notification of the first handover is delayed at second BS120, and notification of the second handover is delayed at the first BS110 while forwarding of user plane data is accomplished between thefirst BS 110 and the second BS 120.

In the procedures of FIG. 13, the first handover from the first BS 110to the second BS 120 is prepared by messages 1301 and 1302. Message 1301may be a handover request from the first BS 110 to the second BS 120,and message 1302 may be a handover request acknowledgement form thesecond BS 120 to the first BS 110, in accordance with 3GPP TS 36.300. Asin the scenario of FIG. 12, message includes the anchor identifier to beforwarded to the next BS of a sequence of handovers.

By sending message 1303 to the UE 10, the first BS 110 then initiatesexecution of the handover. In accordance with 3GPP TS 36.300, message1303 may be a RRC connection reconfiguration message and convey mobilitycontrol information to the UE 10. As illustrated by step 1304, the UE 10may then detach from the cell of the first BS 110 and synchronize to thecell of the second BS 120. The first BS 110 may then send statusinformation to the second BS 120, using message 1305.

As illustrated by message 1306, the first BS 110 may then startforwarding downlink user plane data to the second BS 120.

By sending message 1307 to the second BS 120, the UE 10 then sets up aconnection to the second BS 120. After this, uplink user plane data canbe conveyed directly via the second BS 120 to the gateway 350, asillustrated by messages 1308 and 1309, while downlink user plane datacan be conveyed from the gateway 350 to the first BS 110 and thenforwarded via the second BS 120 to the UE 10, as illustrated by messages1310 to 1312.

In the procedures of FIG. 13, execution of the first handover iscompleted by the second BS 120 sending message 1313 to the first BS 110,to initiate releasing of UE resources at the first BS 110. Message 1313may be UE context release message in accordance with 3GPP TS 23.401 and36.300. Further, the second BS 120 starts the timer of duration T fordelaying notification of the handover to the control node 320.

Upon receiving message 1313, the first BS 110 releases resources relatedto the UE 10, as indicated by step 1314. However, the first BS 110 keepssufficient state information to continue forwarding downlink user planedata of the UE 10. In accordance with the illustrated concepts, thesource BS 110 is configured to continue with packet forwarding for aforwarding time period T_(f) which is longer than the delay time periodT for delaying the notification of the handover to the control node 320.For this purpose, the forwarding timer of duration T_(f) may be startedtogether with the forwarding of user plane data by the source BS 110,i.e., upon sending message 1306. Further, the first BS 110 maintainscontext information allowing for continued signalling with the second BS120 over the inter-BS interface, in particular for forwarding controlplane data between the first BS 110 and the second BS 120.

The second handover from the second BS 120 back to the first BS 110 isthen prepared by messages 1314 and 1315. Message 1314 may be a handoverrequest from the second BS 120 to the first BS 110, and message 1315 maybe a handover request acknowledgement form the first BS 110 to thesecond BS 120, in accordance with 3GPP TS 36.300. The message 1314includes the anchoring identifier which was forwarded to the second BS120 in the handover request of message 1301.

By sending message 1316 to the UE 10, the second BS 120 then initiatesexecution of the second handover. In accordance with 3GPP TS 36.300,message 1316 may be a radio resource control connection reconfigurationmessage and convey mobility control information to the UE 10. Asillustrated by step 1317, the UE 10 may then detach from the cell of thesecond BS 120 and synchronize to the cell of the first BS 110. Thesecond BS 120 may then send status information to the first BS 110,using message 1318.

As illustrated by message 1319, the second BS 120 may then startforwarding downlink user plane data to the first BS 110. Here, it is tobe understood that it is the first BS 110 which in turn forwards suchdownlink user plane data to the second BS 120.

By sending message 1320 to the first BS 110, the UE 10 then again setsup a connection to the first BS 110. After this, uplink user plane datacan be conveyed directly via the first BS 110 to the gateway 350, asillustrated by messages 1321 and 1322.

At step 1323, the first BS 110 detects the formation of a forwardingloop between the first BS 110 and the second BS 120. This isaccomplished on the basis of the anchor identifier received from thesecond BS 120 in message 1314. In particular, the second BS 120 maydetermine that the anchor identifier is the same as provided to thesecond BS 120 in message 1301.

In the scenario of FIG. 13, since the number of forwarding hops islimited to a maximum of two hops, the first BS 110 can remove thedetected forwarding loop by stopping forwarding with respect to thesecond BS 120 and releasing the corresponding resources and context, asindicated in step 1324.

Having removed the forwarding loop, downlink user plane data can beconveyed from the gateway 350 to the first BS 110 and then forwardeddirectly to the UE 10 as illustrated by messages 1325 to 1326.

Execution of the second handover is then completed by the first BS 110sending message 1327 to the second BS 120, to initiate releasing of UEresources at the second BS 120. Message 1327 may be UE context releasemessage in accordance with 3GPP TS 23.401 and 36.300. The message 1327may include an indication of required forwarding loop removal, e.g., inthe form of a flag.

Upon receiving message 1327, the second BS 110 releases resourcesrelated to the UE 10, as indicated by step 1328 and, in response to theindication in message 1327, stops forwarding and releases the resourcesand context associated with the forwarding. The latter procedures may bedelayed by a configurable time interval to account for delays in theforwarded user plane and/or control plane data and to allow forreordering of data packets.

The above concepts of delaying handover notification at the target BSmay also be considered in procedures for releasing a user plane andcontrol plane connection of the UE 10, e.g., an S1 connection. If theabove option of forwarding the control plane data between the BSs isused, also the control plane data associated with the signalling forreleasing the connection may be forwarded in the same manner.Corresponding procedures are illustrated in the exemplary timing diagramof FIG. 14.

The procedures of FIG. 14 involve the UE 10, the first BS 110, thesecond BS 120, the third BS 130, and the control node 320. In theillustrated scenario, it is assumed that the UE 10 performed a firsthandover from the first BS 110 to the second BS 120 and then a secondhandover from the second BS 120 to the third BS 130, so that the UE 10is connected to the third BS 130, but the core network, in particularthe control node 320, is still connected to the first BS 110. That is tosay, the control plane connection between the third BS 130 and thecontrol node 320 is established via the second BS 120 and the first BS110, using forwarding of control plane data over the inter-BSinterfaces.

For sending a request for releasing connection of the UE 10 to thecontrol node 320, e.g., a UE Context Release Request of the S1-MMEinterface, the third BS 130 includes this request into message 1401,which is sent from the third BS 130 to the second BS 120. The second BS120 forwards the request to the first BS 110, using message 1402, andthe first BS 110 forwards the request to the control node 320, usingmessage 1403.

The control node 320 then responds to the request by sending message1404 to the first BS 110. The response of message 1404 may for exampleinclude a UE Context Release Command of the S1-MME interface.

The first BS 110 forwards the response to the second BS 120, asindicated by message 1405, and the second BS 120 forwards the responseto the third BS 130, as indicated by message 1406.

The third BS 130 may then perform releasing of radio resources relatedto the UE 10. This may involve sending message 1407 to the UE 10. Forexample, message 1407 may be a RRC resource release message.

The third BS 130 may then inform the control node 320 of havingcompleted the control plane context release, by including acorresponding notification into message 1408, which is sent from thethird BS 130 to the second BS 120. The second BS 120 forwards thenotification to the first BS 110, using message 1409, and the first BS110 forwards the notification to the control node 320, using message1410. The notification may be a UE Context Release Complete message ofthe S1-MME interface.

The third BS 130 then instructs the second BS 120 to stop forwardingcontrol plane data by including a corresponding command into message1411, which is sent from the third BS 130 to the second BS 120. Thesecond BS 120 forwards the command to the first BS 110, using message1412. In some scenarios, the command may also be included in messages1408 and 1409.

In response to the command, the second BS 120 and the first BS 110 stopforwarding of control plane data over the inter-BS interface and canrelease the corresponding context and resources, as indicated by steps1413 and 1415. At step 415, the third BS 130 may release all resourcesrelated to the UE 10, including the context maintained for theforwarding of control plane data over the inter-BS interface.

In the above concepts involving delaying of the handover notification atthe target BS, cooperation between the involved BSs may be required. Insuch cases, support of the required functionalities may be verified bythe BSs. For example, the support may be negotiated when setting up aconnection via the inter-BS interface or when exchanging the handoverrequest message and handover request acknowledgement messages. Further,information whether a certain neighboring BS supports thefunctionalities or not may also be statically configured in the BSs,e.g., using OAM procedures. If the required functionalities are notsupported by a neighboring BS, the target or source BS may refrain fromapplying the concepts when performing a handover with respect to thisneighboring BS.

If in a sequence of handovers forwarding of user plane data and/orcontrol plane data is performed as described above, and the UE 10 thenperforms a handover from a source BS supporting the requiredfunctionalities to a target BS not supporting the requiredfunctionalities, the source BS may detect that the target BS does notsupport the required functionalities and notify the forwarding BSs ofthe sequence accordingly, so that these BS can stop forwarding andrelease the corresponding resources and context of the inter-BSinterface. In some scenarios, the stop of forwarding and release of thecontext and resources may also be controlled by the forwarding timer andassociated forwarding time period T_(f).

In some situations, a handover may not progress regularly, and radiolink failures may occur. Generally, such failures may be handled inaccordance with existing methods, e.g., as described in 3GPP TS 36.300.

In the case of a radio link failure to a prepared target BS, i.e., ifthe handover to the target BS was prepared by handover request andacknowledgement, but execution fails due to a failure in establishingthe radio link between the UE 10 and the target BS, the target BS cannotify the source BS accordingly so that the source BS can stopforwarding and release the corresponding context and resources of theinter-BS interface. The notification may also be forwarded to otherforwarding BSs of a sequence of handovers for stopping the forwardingand releasing the corresponding contexts and resources. In somescenarios, the stop of forwarding and release of the context andresources may also be controlled by the forwarding timer and associatedforwarding time period T_(f). Further, also the core network 300, inparticular the control node 320, may be notified about the status of theUE 10 after the failure.

Further, also a radio link failure to an unprepared target BS may occur.In this case there is no preceding handover request and acknowledgement,which may occur when the UE 10 moves to the target BS in idle mode andthen attempts connection, e.g., by issuing a service request. In such acase, the control node 320 may release the control plane and user planeconnections to the source BS. The source BS may then notify otherforwarding BSs of a sequence of handovers so that these BS can stopforwarding and release the corresponding resources and context of theinter-BS interface. In some scenarios, the stop of forwarding andrelease of the context and resources may also be controlled by theforwarding timer and associated forwarding time period T_(f).

In the above concepts, a number of criteria may be used to control thedelay of the handover notification from the target BS to the controlnode 320 and the extended forwarding of user plane data and/or controlplane data.

The basic criterion is the delay time period T from the execution of thehandover to the notification of the handover to the control node 320. Bydelaying the notification of the control node 320 until this delay timeperiod T expires, a sequence of multiple handovers may cause only asingle notification to the control node 320.

As a further criterion, a maximum number P_(max) of handovers beforenotifying the control node 320 may be defined, e.g., by implementing acorresponding counter. If the number of handovers in a sequence ofhandovers reaches this maximum number P_(max,) the control node 320 maybe notified even if the delay time period T has not yet expired. In thisway, the number of hops for forwarding user plane data and/or controlplane data can be limited. The counter value may be forwarded from thesource BS to the target BS, e.g., in the UE context.

As a further criterion, a timeout T_(t) on the basis of the total timesince the notification of the last handover to the control node 320 maybe used, e.g., by implementing a further timer. Upon reaching thetimeout T_(t), the control node 320 may be notified even if the delaytime period T has not yet expired. In this way, the notification of thecontrol node 320 is not delayed more than T_(t) even if there aremultiple handovers within the delay time period T.

As already discussed above, it is possible to use explicit signallingfor releasing the context and resources for forwarding on the inter-BSinterface, or to use a timeout after the forwarding time period T_(f)after which the resources and context are released without any explicitsignalling. If the delay time period T is used in connection with themaximum number of handovers P_(max), the forwarding time period T_(f)can be set to be at least T_(f)=T×P_(max). If the delay time period T isused in connection with the timeout T_(t), the forwarding time periodT_(f) can be set to be at least T_(f)=T_(t).

The values for T, P_(max), and T_(t) may differ between different BSs ofthe mobile network. These values may be statically configured in the BS.Further, these values could be dynamically obtained either throughprovisioning over an external interface or through a self learningsystem in the BS, thereby allowing for an effective balancing between onthe one hand handover related signalling load and on the other handforwarding related load on the inter-BS interfaces.

Also when delaying the handover notification at the target BS, it may beuseful to restart the timer or timers and/or the counter of notifiedhandovers when the UE 10 returns to an earlier BS of a sequence ofhandovers, i.e., when a forwarding loop is detected. In this way thesignalling reduction in the case of ping-pong or loop behaviour can befurther improved.

As mentioned above, the anchor identifier may be used for the detectionof forwarding loops. The anchor identifier may be stored in the UEcontext and forwarded to the target BS during handover. Additionally oras an alternative, also identifiers of the BSs of a sequence ofhandovers may be recorded and forwarded to the target BS as the UE 10moves through the sequence. For this purpose, each BS may add itsidentifier to a list and forward the list to the nest target BS, e.g.,in the UE context. This may facilitate detection of a forwarding loop,because the target BS only needs to recognize its own identifier in thelist received from the source BS in order to detect the forwarding loop.

In the above, concepts were described which are based on either delayingthe handover notification in the RAN 100, i.e., at the target BS, or ondelaying the handover notification in the core network 300, i.e., at thecontrol node 320. However, it is also possible to combine these conceptsin the same mobile network. For example, in some domains of the mobilenetwork the delay could be implemented in the RAN 100, and in otherdomains it could be implemented in the core network 300. Further, inview of efficiency, the application of the delay could in each case belimited to specific situations. For example, the delay could be appliedonly for UEs with only non-guaranteed bitrate bearers. This helps tofacilitate network dimensioning, because guaranteed bitrate bearersrequire resource reservation in the transport network which may becomemore complex when extended forwarding of user plane data is applied.Further, the delay could be applied only for UEs which have a handoverrate exceeding a certain threshold. The threshold for the handover ratemay be configurable. For UEs with high handover rate, application of thedelay is specifically beneficial because a considerable reduction of thesignalling load can be achieved.

FIG. 15 shows a flowchart for illustrating a method for handoversignalling. The method may be used for implementing the above conceptsin a node of a mobile network, e.g., in a control node of the mobilenetwork, such as the control node 320 of the above scenarios, or in a BSof the mobile network, such as the target BS 120 or 130, in the abovescenarios.

At step 1510, a handover of the UE, e.g., the UE 10, from a source BS toa target BS is performed. The source BS and the target BS may forexample correspond to a pair of BSs in the above described scenarios,e.g., to the BSs 110 and 120 or to the BSs 120 and 130. 2. If the nodecorresponds to the control node the handover may be notified to thecontrol node by a path switch request from the target BS, such as bymessage 213 of FIG. 2, by message 515 of FIG. 5, or by message 613 or630 of FIG. 6. If the node corresponds to the target BS, suchnotification of the handover is not needed, because the target BS itselfmay determine that execution of the handover is completed.

At step 1520, the node of the mobile network starts a timer, whichexpires at step 1530. The timer has the purpose of delaying notificationof the handover by the node. The duration of the timer may beconfigurable. For example, the node may receive configuration data,e.g., through a OAM procedure, and control the duration of the timer onthe basis of the configuration data.

At step 1540, after expiry of the timer, the node sends controlsignalling for initiating path switching to a data path connecting thetarget BS to a gateway of the mobile network. If the node corresponds tothe control node, the control signalling may comprise a modify bearerrequest to the gateway. Examples of such modify bearer requests aremessage 218 of FIG. 2 and message 520 of FIG. 5. If the node correspondsto the target BS, the control signalling may comprise a path switchrequest to a control node of the mobile network, e.g., to the controlnode 320. Examples of such a path switch request are message 716 of FIG.7 and message 818 of FIG. 8.

Before expiry of the timer but after setting up a connection between theUE and the target BS, if the node corresponds to the target BS, thetarget BS may send uplink control plane data via the source BS to thecontrol node. Further, the target BS may receive downlink control planedata via the source BS from the control node. Examples of suchforwarding of control plane data are illustrated in FIG. 9. Also, if thenode corresponds to the target BS, if a need to transmit control planedata between the target BS and the control node arises, the target BSmay send the control signalling irrespective of expiry of the timer.

Before expiry of the timer but after setting up a connection between theUE and the target BS, the target BS may receive downlink user plane datafrom the source BS and forward the received downlink user plane data tothe UE, e.g., as illustrated by messages 210, 211, 212, of FIG. 2,messages 511, 512, 513, 514 of FIG. 5, messages 610, 611, 612, 626, 627,628, 629 of FIG. 6, messages 710, 711, 712 of FIG. 7, messages 811, 812,813, 814 of FIG. 8, messages 1213, 1214, 1215, 1216, 1217 of FIG. 12, ormessages 1310, 1311, 1312 of FIG. 13.

The method may also include a further handover from a further BS to thesource BS, e.g., the handover from the first BS 110 to the second BS 120in the scenario of FIG. 3. That is to say, there may be a sequence ofhandovers between multiple BSs, and these handovers may take placebefore expiry of the timer. In such scenarios, there may be forwardingof user plane data and/or control plane data the further BS and thetarget BS via the source BS, e.g., as illustrated in FIG. 3. The nodemay then obtain information for identifying the further base station,e.g., by means of the anchoring identifier described in connection withFIGS. 12 and 13 or by means of BS identities as described in connectionwith FIG. 6. On the basis of the obtained information, the node may thendetect the formation of a forwarding loop, i.e., that the UE returned toan earlier BS in the sequence of handovers.

In some scenarios, forwarding of user plane data and/or control planedata may also be accomplished directly between the further BS and thetarget BS, e.g., as illustrated in FIG. 4.

FIG. 16 schematically illustrates exemplary structures for implementingthe above-described concepts in a control node, e.g., in the controlnode 320 of the above scenarios. As mentioned above, the control nodemay be implemented as an MME.

In the illustrated structure, the control node includes a controlinterface 328 for communicating with BSs of the mobile network, e.g.,implemented as eNBs. The control interface 328 may for example beimplemented as the S1-MME interface. Further, the control node includesa gateway interface 326 for communicating with a gateway of the mobilenetwork, e.g., an SGW. For example the gateway may correspond to thegateway 350 of the above scenarios. The gateway interface 326 may beimplemented as the S11 interface.

Further, the control node includes a processor 325 and a memory 330. Theprocessor 325 is coupled to the gateway interface 326 and to the controlinterface 328, and the memory is coupled to the processor 325. Thememory 160 may include a Read Only Memory (ROM), e.g., a flash ROM, aRandom Access Memory (RAM), e.g., a Dynamic RAM (DRAM) or Static RAM(SRAM), a mass storage, e.g., a hard disk or solid state disk, or thelike. The memory 330 includes suitably configured program code to beexecuted by the processor 325 so as to implement the above-describedfunctionalities of the control node. More specifically, the memory 330may include a handover signalling module 332 for performing theabove-described handover related signalling with respect to BSs and thegateway. This may in particular include the delaying of the handovernotification to the gateway, using the timer associated with the delaytime period T. Further, the memory 330 may include a control module 334for performing various control procedures, e.g., related to connectionsetup and mobility of a UE.

It is to be understood that the structure as illustrated in FIG. 16 ismerely schematic and that the control node may actually include furthercomponents which, for the sake of clarity, have not been illustrated,e.g., further interfaces or additional processors. Also, it is to beunderstood that the memory 330 may include further types of program codemodules, which have not been illustrated. For example, the memory 330may include program code modules for implementing typicalfunctionalities of a control node, e.g., known functionalities of anMME. According to some embodiments, also a computer program product maybe provided for implementing concepts according to embodiments of theinvention, e.g., a computer-readable medium storing the program codeand/or other data to be stored in the memory 330.

FIG. 17 schematically illustrates exemplary structures for implementingthe above-described concepts in a BS, e.g., the BS 110, 120 or 130 ofthe above scenarios. As mentioned above, the BS may be implemented as aneNB.

In the illustrated structure, the BS includes a radio interface 152, agateway interface 154, an inter-BS interface 156, and a controlinterface 158. The radio interface 152 may be used for connecting to theUE, e.g., the UE 10. The gateway interface 154 may be used for conveyinguser plane data to or from a gateway of the mobile network, e.g., thegateway 350. The inter-BS interface 156 may be used for connecting toother BSs, e.g., for the purpose of performing handover relatedsignalling as described above or for the purpose of forwarding userplane data and/or control plane data. The control interface 158 may inturn be used for conveying control plane data to or from a control nodeof the mobile network, e.g., the control node 320. The radio interfacemay be implemented as the Uu radio interface, the gateway interface 154may be implemented as the S1-U interface. The inter-BS interface 156 maybe implemented as the X2 interface, and the control interface 158 may beimplemented as the S1-MME interface.

Further, the gateway includes a processor 150 coupled to the interfaces152, 154, 156, 158 and a memory 160 coupled to the processor 150. Thememory 160 may include a ROM, e.g., a flash ROM, a RAM, e.g., a DRAM orSRAM, a mass storage, e.g., a hard disk or solid state disk, or thelike. The memory 160 includes suitably configured program code to beexecuted by the processor 150 so as to implement the above-describedfunctionalities of the BS. More specifically, the memory 160 may includea handover signalling module 162 for performing the above-describedhandover related signalling with respect to the control node and otherBSs. This may in particular include the delaying of the handovernotification to the control node, using the timer associated with thedelay time period T, or performing forwarding of user plane data and/orcontrol plane data to or from other BSs. Further, the memory 160 mayinclude a control module 164 for performing various control procedures,e.g., control of handover related procedures such as establishment orreleasing of connections or control of radio resources assigned to theUE.

It is to be understood that the structure as illustrated in FIG. 17 ismerely schematic and that the BS may actually include further componentswhich, for the sake of clarity, have not been illustrated, e.g., furtherinterfaces or additional processors. Also, it is to be understood thatthe memory 160 may include further types of program code modules, whichhave not been illustrated. For example, the memory 160 may includeprogram code modules for implementing typical functionalities of a BS,e.g., known functionalities of an eNB. According to some embodiments,also a computer program product may be provided for implementingconcepts according to embodiments of the invention, e.g., acomputer-readable medium storing the program code and/or other data tobe stored in the memory 160.

As can be seen, concepts as described herein may be used to reduce thehandover related signalling load in the mobile network, in particular ata control node such as an MME of the EPC or at a gateway such as the SGWof the EPC. This in turn may allow for a more efficient deployment ofthe mobile network and for cost saving or increased capacity, e.g., inview of the number of UEs which can be served by a given infrastructureof the mobile network. The handover related signalling load may forexample be reduced if a sequence of handovers takes place before thedelay time period expires, so that only the last handover of thesequence needs to be signalled. Further, the handover related signallingload may be reduced if the UE returns to a previous BS of a sequence ofhandovers before the delay time period expires so that handovernotification can be omitted. Further, the handover related signallingload may be reduced if a connection to the UE is released before thedelay time period expires so that handover notification can be omitted.If the delay is implemented in the control node, the concepts can beimplemented without significant impact on the RAN which only needs tosupport extended forwarding between the BSs. If the delay is implementedin the target BS, the concepts can be efficiently implemented withoutfurther impact on the core network.

It is to be understood that the concepts as explained above are merelyexemplary and susceptible to various modifications. For example, theconcepts may be applied in various types of mobile networks which arebased on different types of radio access technology. Further, it is tobe understood that the illustrated nodes may each be implemented by asingle device or by multiple devices. Moreover, the concepts may beimplemented by dedicated hardware and/or by software to be executed by amultipurpose processor in one of the involved nodes.

1. A method of signalling a handover of a user equipment in a mobilenetwork, the method comprising: in response to a handover of the userequipment from a source base station to a target base station, startinga time by a node of the mobile network; and after expiry of the timer,sending from the node control signalling for initiating switching to adata path which connects the target base station and the gateway.
 2. Themethod according to claim 1, wherein the node is a control node of themobile network and the handover is notified to the control node by apath switch request from the target base station; and wherein thecontrol signalling comprises a modify bearer request to the gateway. 3.The method according to claim 1, wherein the node is the target basestation; and wherein the control signalling comprises a path switchrequest to a control node of the mobile network.
 4. The method accordingto claim 3, comprising: before expiry of the timer but after setting upa connection between the user equipment and the target base station,sending by the target base station uplink control plane data via thesource base station to the control node.
 5. The method according toclaim 3, comprising: before expiry of the timer but after setting up aconnection between the user equipment and the target base station,receiving by the target base station downlink control plane data via thesource base station from the control node.
 6. The method according toclaim 3, comprising: in response to a need for transmitting controlplane data between the target base station and the control node, sendingby the target base station said control signalling irrespective of theexpiry of the timer.
 7. The method according to claim 1, comprising:before expiry of the timer but after setting up a connection between theuser equipment and the target base station, receiving by the target basestation downlink user plane data from the source base station andforwarding the received downlink user plane data to the user equipment.8. The method according to claim 1, comprising: a further handover froma further base station to the source base station.
 9. The methodaccording to claim 8, comprising: forwarding of user plane data and/orcontrol plane data via the source base station between the further basestation and the target base station.
 10. The method according to claim9, the node obtaining information for identifying the further basestation; and on the basis of the information, detecting by the nodeformation of a forwarding loop.
 11. The method according to claim 9,forwarding of user plane data and/or control plane data directly betweenthe further base station and the target base station.
 12. The methodaccording to claim 1, comprising: the node receiving configuration data;and on the basis of the configuration data, controlling by the nodeduration of the timer.
 13. A node of a mobile network, comprising: aninterface for signalling a handover of a user equipment; and aprocessor, wherein the processor is configured to: in response to ahandover of the user equipment from a source base station to a targetbase station, start a timer, and after expiry of the timer, control theinterface to send control signalling for initiating switching to a datapath which connects the target base station and the gateway.
 14. Thenode according to claim 13, wherein the node is a control node of themobile network.
 15. The node according to claim 13, wherein the node isthe target base station.
 16. The node according to claim 15, wherein:the control signalling comprises a path switch request to a control nodeof the mobile network; and before expiry of the timer but after settingup a connection between the user equipment and the target base station,the processor is configured to send station uplink control plane datavia the source base station to the control node.